When .alpha.,.beta.-unsaturated aliphatic aldehydes such as acrolein or methacrolein are oxidized in the liquid phase with oxygen or an oxygen-containing gas such as air, a mixture of products is obtained in the resulting oxidate solution. Based on the total weight, the oxidate solution will generally contain unreacted aldehyde of from 20 to 70 weight per cent, and from 40 to 10 weight percent of the corresponding acid product and peroxide (peroxy) compounds of the unsaturated aldehyde feed materials and peroxide (peroxy) compounds of the unsaturated acid product. The unsaturated peroxide (peroxy) compounds co-produced during the autoxidation of acrolein and methacrolein to acrylic acid and methacrylic acid would be peracrylic acid and acrolein monoperacrylate and permethacrylic acid and methacrolein monopermethacrylate respectively.
The present invention relates to a process for the ruthenium and/or osmium catalyzed conversion or decomposition of the above described unsaturated peroxide compounds formed from the autoxidation of the unsaturated aldehydes (acrolein or methacrolein). The decomposition is carried out after the autoxidation stage is completed. Employment of the ruthenium or osmium catalysts during the oxidation step of the aldehyde does not successfully provide an in situ conversion of the intermediate peroxide compounds to the desired acid product and will cause undesirable and excessive polymerization of the acid product. Thus, while the oxidate product solution of the autoxidation of an aldehyde containing the carboxylic acid product, i.e., acrylic or methacrylic acid, may be treated directly according to the process of this invention, the process to decompose the peroxides is carried out after the aldehyde has been oxidized and the intermediate peroxide compounds formed. Conversion of the peroxide compounds according to this invention provides for a high selectivity to the acid and for the recovery of large percentages of the desired unsaturated carboxylic acid (acrylic or methacrylic) as well as a minimum amount of polymer formation, resulting in high overall yield of the acid from the particular original unsaturated aldehyde as compared to prior art processes, including straight thermal decomposition of peroxide compounds.
U.S. Pat. No. 3,253,025 discloses a process for the preparation of unsaturated aliphatic acids such as methacrylic, by subjecting an oxidate containing peroxides to treatment with a hot liquid solvent of p-toluene sulfonic acid and an alcohol to decompose the peroxides to the acid. However, in this process relatively large percentages of polymer are produced.
An article by Benjamin Phillips et al, Journal of the American Chemical Society, Vol. 76, pp. 5982-5986, 1957 shows the preparation of peracetic acid by the autoxidation of acetaldehyde and with peracetic acid and acetaldehyde monoperacetate as intermediates. At temperatures above 20.degree. C. the acetaldehyde monoperacetate decomposes readily yielding acetic acid. Straight thermal decomposition of the .alpha.,.beta.-unsaturated aliphatic aldehyde peroxy intermediates, such as methacrolein monopermethacrylate and acrolein monoperacrylate does not selectively give high yield of the respective acrylic and methacrylic acids and in addition the rate of reaction is low.
To date no commercially successful process has been developed for the preparation of acrylic acid or methacrylic acid involving the autoxidation of the corresponding .alpha.,.beta.-unsaturated aldehyde and the conversion of the co-produced peroxide intermediate.
The acrylic and methacrylic acid products obtained by the process of this invention have many known commercial uses, particularly for the preparation of esters such as methyl methacrylate and as monomers for polymer formation.
A particular advantage of the process of the present invention is the discovery that catalytic amounts of ruthenium or osmium metal, organic and inorganic ruthenium or osmium compounds or mixtures thereof permit the respective peroxide (peroxy) compounds to be selectively decomposed or converted to the acid, e.g., methacrolein monopermethacrylate to methacrylic acid providing an overall process advantage in the liquid phase autoxidation of the aldehyde.